1. Field of Invention
The field of invention relates to apparatus and methods for determining whether heat sensors are within permissible tolerances for their intended purposes
2. Description of Related Art
Heat sensors may be in the form of thermocouples, resistance temperature detectors, thermistors, temperature transmitters, bi-metal thermometers, mercury glass thermometers, or other such similar devices. There is widespread need for accurate heat sensors in industry and in the military, including such applications as missile silos, tanks, battleships, aircraft, chemical processes, steel making, plastic injection molding, autoclaves, hot water tanks, medical appliances, automobiles, and domestic appliances, such as ovens, refrigerators, and microwaves.
For heat sensors to perform their intended function, it is critical that they be accurate within a predetermined tolerance of a prescribed set point temperature, and that they be tested periodically for accuracy. Devices for such testing are available in the prior art, and are referred to by some suppliers as temperature calibration instruments.
The prior art device closest to the subject invention known to Applicant is his own invention, marketed by ThermaCal, Inc., and identified as "Model 18 Calibration Cool/Heat Source." In this device, an elongated, rectangular metal block is provided with a longitudinal cylindrical bore hole, which performs as an oven, having a diameter sufficient for a slip fit with the pencil-shaped cylindrical probe being tested. The metal block is preheated to a set point temperature, whereinafter the probe is inserted in the bore hole and the temperature for the metal block sensed by the probe is recorded or observed on suitable equipment designed for this purpose. If the sensed temperature is within the prescribed tolerance for the probe being tested, the probe may be returned to service. If not, either the probe may be scrapped, or the heat source mechanism being monitored by the probe may be recalibrated to function properly within the predetermined actual known tolerance of the probe.
It is customary for each block to have a plurality of bore holes, each of a different diameter, in order to accommodate probes of different diameters. The bore hole diameters are usually not less than 0.0625 inches, and 90 percent of the commercial market can be serviced with bore hole diameters between 0.0625 and 0.625 inches.
The prior art device just discussed is limited to testing cylindrical probes. The industrial market for temperature sensor, however, is highly diverse. This diversity has resulted in the design of many application-specific temperature sensing probes, such as those illustrated in prior art FIGS. 1, 2, and 3. Probe 1 of FIG. 1 is a Type J thermocouple probe used in plastic molding machines, wherein the ferrule 2 provides a seal against hot high pressure plastics. Because of its enlarged configuration, the probe cannot be tested in prior art devices such as the ThermaCal, Inc., Model 18 comparator described above. Some Type J thermocouples, therefore, must be tested by other means, such as by hot oil baths, which are expensive, cumbersome, and not easily portable.
Probes for domestic appliances, FIG. 2, such as ovens, hot water heaters, refrigerators, dishwashers, gas-fired space heaters and the like, are mass produced, low cost, temperature sensors. A resistance temperature detector (RTD) substrate 3 is inserted into an open-ended tube 4. Then the tube ends 5 and 6 are swaged or clamped shut about the substrate. The technique is inexpensive, and the finished product is not liquid-tight. Therefore, this class of probe cannot even be tested in an oil bath. Furthermore, because of the irregular shape resulting from swaging, these probes cannot be effectively tested in the ThermaCal, Inc., Model 18 type comparator.
A pharmaceutical in-pipe probe 7, including threaded nut 8 and sleeve 9, such as shown in FIG. 3, like the FIG. 2 probe for domestic appliances, cannot be tested in a Model 18 type comparator because of its irregular shape. Because the sleeve 9 is not sealed against fluid penetration, testing in a hot oil bath is a very delicate procedure.
Prior art comparators with multi-hole metal blocks also have a source of error referred to as hole-to-hole non-uniformity of temperature. Usually this error is a few tenths of a degree Celsius, which is bothersome in many applications since a hole at the top of the block may not be at the same temperature as a hole at the bottom of the block.
Heretofore, only temperature sensors having cylindrical probes could be efficiently tested in prior art testing devices, such as Applicant's Model 18 comparator. Now, for the first time, irregularly shaped temperature sensors, such as shown in FIGS. 1, 2, and 3, can be efficiently and inexpensively tested in Applicant's subject inventive comparator. This fact is particularly significant with respect to inexpensive domestic appliance temperature sensing probes, such as shown in FIG. 2. Because these probes can now be accurately tested, they can be used in more sophisticated mechanisms that require extreme accuracy in temperature control. Thus, Applicant's inventive comparator has opened a wide range of new markets for low cost, previously untestable, temperature sensors.